Anthropometry and Change in Body Composition in Obesity
Waist circumference is commonly used to assess change in abdominal obesity. Changes in waist circumference are associated with changes in visceral fat in response to diet and/or exercise weight loss (Fig. 5) (35,50,52). It is reported that a 1-cm reduction in waist circumference corresponds to a 4% reduction in visceral fat; however, there was a substantial amount of variance (standard deviation = 4%) in this relationship. The variation in this association is in part due to changes in subcutaneous fat and/or lean mass that mask the ability of waist circumference to accurately distinguish changes in abdominal tissues.
When examining the utility of WHR to estimate changes in body composition, a principal limitation of a ratio score becomes clear. Owing to the nature of ratio scores, changes in the ratio could be due to alterations in the numerator (waist) or the denominator (hip). For example, a reduction in the WHR after an exercise intervention could be due to reductions in the waist circumference or increases in the hip circumference due to increases in lower body muscle mass. Similarly, larger reductions in the hip circumference relative to the waist could result in no change or even an increase in the WHR despite significant diet- and/or exercise-induced weight loss. Furthermore, as mentioned before, reductions in waist or hip circumference with weight loss could be a consequence of reductions in fat or lean mass. Thus, a change or lack of change in the ratio score is difficult to interpret. Therefore, changes in WHR may not necessarily reflect changes in abdominal adiposity. This is reinforced when examining the relationship between corresponding changes in WHR and visceral fat. Unlike waist circumference, changes in WHR are not consistently associated with corresponding changes in visceral fat (Fig. 6) (35,50,52). A primary example of the limits inherent to the interpretation of WHR when trying to assess body composition change in obesity studies is reflected in Fig. 7. In response to weight loss due to a negative energy balance, the mobilization of fat is rarely restricted to the abdomen. In fact, two-thirds of the fat loss in our studies (42-45) was seen to be from sites outside of the abdominal region (Fig. 8).